Conventional incandescent light bulbs have a glass envelope which is evacuated or is filled with an inert gas such as argon and/or nitrogen. A thin filament of tungsten is suspended inside the envelope between a pair of electrical leads. Light is produced by passing an electric current through the filament which is heated by the current passing through it until it glows brightly, a process called “incandescence”. Filament temperatures on the order of about 4,500 degrees Fahrenheit (2,500 degrees Celsius) are typical. Incandescent light bulbs are a relatively inefficient way of converting electrical power which is typically measured in Watts, into light which is typically measured in Lumens. The “efficiency” of a lamp is generally expressed according to the amount of visible light the lamp produces as measured in units called “lumens”, divided by the electrical power, measured in “watts”, required to operate the lamp. A lamp with a high ratio of lumens per watt is more energy efficient than one with a lower output of lumens of light per watt of electrical energy consumed. Of the total amount of electrical energy they consume, incandescent lamps convert a much higher percentage of that energy into heat than visible light. Incandescent lamps also have relatively short normal operating lives. After only about 750 to 1,000 hours enough tungsten evaporates from the filament of an incandescent lamp that the filament can no longer support its own weight, causing the lamp to “burn out” as a result of breakage of the filament.
A halogen lamp is an improved type of incandescent lamp. Its tungsten filament is enclosed in a low-volume, gas-filled envelope of quartz. The envelope and the filament are so close to one another that the envelope would melt if it were of ordinary glass. The gas within the envelope is a halogen. At the high normal operating temperatures of a halogen lamp, the gas combines with tungsten that has vaporized off the filament and re-deposits the tungsten back onto the filament, thus both lengthening its life allowing the filament to operate at a significantly higher temperature and thus glow more brightly than an ordinary incandescent bulb. As a result, halogen lamps produce more useful light per unit of electrical power applied to the lamp, i.e. more lumens per watt than a normal incandescent lamp. However, due to their high operating temperature, halogen lamps also waste a large amount of energy that is given off as heat.
Gas discharge lamps of various kinds are also well-known in the prior art. These too include a gas-filled envelope but not have a filament. A fluorescent lamp one type of gas discharge lamp that is widely used. The glass envelope in fluorescent lamp is a typically a glass tube. A small amount of mercury and an inert gas, such as argon, are sealed inside the tube under very low pressure. The inside wall of the tube is coated with a phosphor powder. Each one of a pair of electrodes located at opposite ends inside the tubular glass envelope is wired to a fixture which contains an electrical circuit called a “ballast” that generates a high voltage between the electrodes. That voltage causes electrons to flow through the gas between the electrodes and vaporizes the mercury in the tube. Electrons and mercury atoms collide, raising electrons to higher energy levels. Photons are released as the electrons return to a lower original energy level following those collisions thereby creating light, much of it being invisible ultraviolet (“UV”) light, rather than useful visible light. However, when these photons strike the phosphor coating inside the tube, the phosphor coating releases light within the visible range of the spectrum through a process called “phosphorescence.” Because they convert what would otherwise be invisible UV light into useful visible light, fluorescent lamps are typically much more energy efficient than incandescent lamps.
LEDs produce light by a completely different mechanism than incandescent or gas discharge lamps. An LED is a semiconductor device, namely a diode junction between a p-type semiconductor material and n-type semiconductor material. As an electric current is passed in the forward direction across the p-n junction of an LED, photons are given off as electrons making up the flow of current change their energy levels, thus producing light. This process, called electroluminescence, is an efficient way of generating light from electricity, particularly in comparison to incandescent bulbs and many other types of lamps. However, it is not a process which results in 100% conversion of electrical energy into light. A significant fraction of the energy represented by the electric current flowing through an LED generates heat rather than light. If sufficient amounts of heat are not carried away from the area of the p-n junction at a sufficient rate, the operating temperature of the LED can quickly rise to an unacceptably high temperature which could cause the LED to fail prematurely. Thus, unlike incandescent bulbs and certain other technologies such as high intensity discharge (HID) lamps, which not only tolerate, but actually require, extreme temperatures in order to generate light, LEDs are relatively intolerant of high temperatures, particularly if one desires to maximize the operating life if the LED.
Early LED devices were not capable of producing light in amounts sufficient for general illumination or architectural illumination. They were used mainly as glowing indicators in electronic and consumer devices. However, as a result of advancements in LED technology, LEDs of sufficient light output for flashlights, lanterns and even general and architectural lighting devices have now been available for several years and the technology continues to advance providing new generations of LEDs having greater lumen output, higher efficiency and lower cost than earlier generations. There has been considerable interest in developing LED lighting modules and luminaires which exploit these improvements in LED technology to provide energy cost savings in general and architectural lighting applications. The enormous investment represented by luminaires and light fixtures which are already existing and installed in the field were designed for operation with an incandescent, fluorescent, gas discharge or other conventional type of lamp, has generated considerable interest in developing LED lighting devices which incorporate high intensity LEDs and can be retrofitted into an existing style of light fixture or luminaire as a substitute for a replaced lamp of some other type. However, due in significant part to the inherent intolerance of high temperatures which is characteristic of LEDs, such efforts have met with only limited success.
One approach has been to provide LED luminaires with substantial vent openings which allow air exchange between the interior of the luminaire and the external environment. While vent opening are frequently present in many existing fixtures or luminaires, their sizes and locations are typically not adequate to provide sufficient air exchange to avoid overheating LEDs to a point which at least shortens their operating life. Enlarging and/or relocating vent openings to provide more air flow is not always possible or desirable. By their nature, vent openings can allow for intrusion of dirt, water and/or insects which can damage a fixture or reduce its light output.
As exemplified for example by U.S. Pat. Nos. 7,438,440 and 7,494,248 another approach to dealing with the heat sensitivity of LEDs in luminaires and light fixtures for general and architectural lighting applications has been to connect one or more heat pipes in a thermal path between one or more of the LEDs and a heat sink located exterior to the housing of the fixture or luminaire so as to conduct heat rapidly away from the LED to the external environment. While effective from a thermal management standpoint, fixtures and luminaires constructed in this manner tend to be bulky, complex and relatively expensive to manufacture. Space constraints and the need to modify an existing fixture or luminaire to accommodate the routing of heat pipes make such an approach less than ideal for retrofit applications.